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### Topic: Which capacitor (Read 7057 times)previous topic - next topic

#### dannydebont

##### Nov 03, 2013, 01:42 pm
Hi Guys,

I made a copy of Kevin Darrah's manual input to shift registers. My L.E.D's are jumping all over the place when I give the 74HC595B1 manual input. I tried putting a capacitor over the switches (in an attempt to cancel the bouncing) but now it is even worse.

My question is this: How would I calculate which capacitor is needed to prevent this bouncing of the L.E.D's. Google has too many conflicting formulas. I am familiar with the series and parallel connections of capacitors, but I need some formula for getting the right capacitor for any circuit.

#### fungus

#1
##### Nov 03, 2013, 02:25 pm
I looked my crystal ball but I can't see what you're actually doing from here...

#### dannydebont

#2
##### Nov 03, 2013, 02:30 pm
That is because you are using a crystal ball ... It is clouding your vision ... In a world without crystal balls "asking" normally does the trick ... ...

#### fungus

#3
##### Nov 03, 2013, 04:28 pm
I still have no idea what you're doing - where you're putting the capacitors, what values you've tried, how your 'switches' are wired, etc.

#### TomGeorge

#4
##### Nov 03, 2013, 11:37 pm
Hi Danny, can  you post a copy of your circuit diagram and post a copy of your sketch.
http://forum.arduino.cc/index.php/topic,148850.0.html

Tom.....
Everything runs on smoke, let the smoke out, it stops running....

#### Grumpy_Mike

#5
##### Nov 03, 2013, 11:48 pm
Quote
How would I calculate which capacitor is needed to prevent this bouncing of the L.E.D's

Use the formular Time constant = R x C

#### cjdelphi

#6
##### Nov 06, 2013, 05:37 am
Flickering would indicate a loose connection maybe on the latch or clock..

But a 0.1uf cap should be plenty between 5v pin and gnd pin....

#### dannydebont

#7
##### Nov 09, 2013, 08:53 pm
Hi Guys,

Sorry for the late reply ... I have included a schematic and a Fritzing file. I have now added a de-bouncing circuit and it works absolutely NOT ... NOTHING ... NADA ...

Please give me some pointers ...

Thank You

Danny

#### Paul__B

#8
##### Nov 09, 2013, 11:02 pm
Try raising the resistors on the HC595 LEDs to 1k, put a 100 nF capacitor across Vcc and ground on each (I am assuming you are using a breadboard) and  a 47 µF capacitor across the supply as well.

#### TomGeorge

#9
##### Nov 09, 2013, 11:08 pm
Hi,  Danny, what are you using as your power supply.
Have  you measured the voltage at the various switched input pins to check that they are getting 5V when the switch is activated.

Tom...
Everything runs on smoke, let the smoke out, it stops running....

#### dannydebont

#10
##### Nov 10, 2013, 10:57 amLast Edit: Nov 10, 2013, 11:05 am by dannydebont Reason: 1
Hi,

I am using the power from an Arduino Uno's 5V and GND pins ... I still need to hook it up to be able to work from a 9v (or other) battery ... ( This project is to teach underprivileged school children (and myself of course   ) so it  needs to work of a battery ) ...

The other thing I noticed is that when I take an L.E.D (that is lying around) and hold it against the leg of the DATA PIN or the leg of the LATCH PIN (on the actual 74HC595) while pressing their respective buttons (switches) they are very dim, but when I do the same to the LATCH pin, it is brighter ... Don't think it is serious, just thought I'd mention it ...

I will try the 47uF capacitor and changing the resistors on the output to the L.E.D's ...

Thanks to everyone assisting me ...

Have a nice day

Danny

ps: O yes, all L.E.D's are 3mm

#### TomGeorge

#11
##### Nov 10, 2013, 11:43 am
Hi, I take it that you do not have a multimeter of any sort to measure voltages.
You will need one to help with development of your project.
Even a cheap chinese unit will suffice, a long as it can measure DC/AC , resistance and even test diodes.
The arduino 5V pin will not be able to power those LED's, you will need a separate supply to power the LEDs.

Tom....
Everything runs on smoke, let the smoke out, it stops running....

#### Paul__B

#12
##### Nov 10, 2013, 12:02 pm
Of course the LED diameter has nothing to do with the electrical characteristics.  XD

My suggestion to put a 100 nF (0.1 µF) capacitor across Vcc and ground on each 74HC595 chip may prove to be the most important one.   Digital ICs can oscillate if not correctly bypassed.

The Arduino has a limited capacity to supply current, if the 74HC595 chips managed (though unlikely) to draw sufficient current to exceed this, things definitely would get unreliable.

#### Grumpy_Mike

#13
##### Nov 10, 2013, 02:08 pm
Start off just trying to get one shift register to work only then try three.
The circuit is some what unconventional although nothing strikes be as wrong.
I agree with Tom, get a meter and measure that the voltage on the actual shift register pins is going up and down as expected, up being greater than 4V and down being less than 0.7V.

#### dannydebont

#14
##### Nov 10, 2013, 06:02 pm
O.k Guys,

I made the changes as suggested (see included jpg and Fritzing file ) ...

Is the circuit at least correct?  :~

Danny ...

p.s: I disconnected the other two shift registers  =( ... And yes ... I'll get a bloody multimeter ...

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